COMPUTER AIDED ESTIMATION OF MOLECULAR WEIGHT AND LONG CHAIN BRANCHING DISTRIBUTION IN FREE RADICAL POLYMERIZATION

Abstract

A robust method for calculating molecular weight and long chain branching distribution in free radical polymerization is proposed in this work. This method is based on direct integration of large nonlinear integro-differential equations system describing the conservation of dead polymer and live radicals in the reactor. A fairly general kinetic mechanism was employed to describe the complex kinetics of homo- and co- polymerization in presence of branching reactions such as transfer to polymer and terminal double bond polymerization. To simplify calculations and reduce the order of prohibitively large nonlinear system the long chain hypothesis in addition to quasi steady state and continuous variable approximation were applied to live radical mass balances. This method was employed to calculate molecular weight and long chain branching distribution of poly(p-methyl styrene) and poly(vinyl acetate) produced by bulk homopolymerization in a batch reactor. Assumptions are justified by comparing calculated distributions, with experimental data and solutions obtained without invoking any assumption. Number and weight average molecular weight of the calculated molecular weight distributions are in excellent agreement with experimental data. This methodology is extended to branched copolymers to obtain the total weight molecular weight distribution by assuming that the respective copolymer composition distribution is uniform. In all cases, the obtained number and weight average molecular weight of calculated molecular distributions are in excellent agreement with obtained independently by moments' method. Results are presented, showing the effect of branching reactions on molecular weight distribution of copolymers. It is believed that present method can be applied to other free radical polymerization systems, to calculate the molecular weight and long chain branching distribution, leading thus to a more rational design of polymerization reactors.